Project Summary Lrp5 is a positive regulator of bone formation in vertebrates. It is generally assumed that it fulfills this function as a coreceptor for Wnt proteins. However, disrupting canonical Wnt signaling in osteoblasts does not affect bone formation thus raising the testable hypothesis that Lrp5 uses other and/or additional means to regulate bone formation. While testing this hypothesis we have generated a large body of preliminary data indicating that in absence of Lrp5 there is a severe increase in circulating levels of peripherally produced serotonin, which known to be produced by the enterochromaffin cells of the gut (duodenum). Based on this body of preliminary data we hypothesize that, in mice and men, the bone diseases triggered by loss- or gain-of-function mutations in Lrp5 are endocrine diseases originating from the gut and mediated by serotonin. To demonstrate the validity of our hypothesis we propose to pursue the following specific aims: - To generate and analyze mice lacking Lrp5 in either osteoblasts or duodenum to determine the cellular origin of the bone phenotype caused by Lrp5 inactivation. - To generate and analyze mutant mice lacking Tph1 in osteoblasts or duodenum to determine whether serotonin acts on osteoblast in an autocrine or endocrine manner. - To determine through genetic means whether a gain-of-function mutation in Lrp5 affects bone mass in a serotonin-dependent manner. - To use genetic assays to identify serotonin signaling receptors in osteoblasts - To determine genetically whether CREB is the transcription factor acting downstream of serotonin in osteoblasts.